The invention generally relates to methods of making foamed materials from a blend of thermoplastic polymer materials employing various blowing agents.
Methods for making conventional foamed materials, such as for example thermoplastic materials, have long been known. These methods have focused primarily on employing chemical and physical blowing agents that include, for example, azos, various volatile organic compounds (VOCs), and chlorofluorocarbons (CFCs). The chemical blowing agents typically decompose at a critical temperature and release a gas such as nitrogen, carbon dioxide, or carbon monoxide. The physical blowing agents typically are dissolved in the polymer material and then precipitated to form a foamed structure. Conventional foaming processes are believed to be becoming increasingly undesirable. For example, the use of these organic compounds has received heightened scrutiny due to potential environmental risks associated with the same. Moreover, conventional foaming processes often disadvantageously produce voids or cells within the materials that are relatively large, for example approximately 100 microns or greater, along with relatively wide variances of void fraction percentages. The number of voids per unit volume of the polymeric material may be relatively low and the distribution of cells through out the polymeric material is often non-uniform. The typical embodiment of an improved foaming process would be to use carbon dioxide as a physical blowing agent to generate a polymeric foam which contains a uniform distribution of cells (with sizes ranging from about 10 microns to submicron size) defined herein as microcellular.
Semicrystalline polymers present various processing challenges, particularly with respect to producing microcellular foams via a continuous processing. Structural foams, along with other conventional foams, typically proceed commercially with physical blowing agents and semicrystalline polymers. Nonetheless, problems with these foams tend to stem from the small size of the cell walls, and a small defect in the cell wall is often a significant weak point in the structure leading to the observed problems. Producing various foamed semicrystalline thermoplastic polymers such as (poly)vinylidene fluoride in a continuous process has often been observed to be problematic. It has been observed that as the material exits the nozzle of a foaming system, the extrudate often becomes brittle and breaks up into fine powder. Although not wishing to be bound by theory, it is believed that the semicrystalline nature of polymer causes the extrudate to break into small particles when the blowing agent is released. This tends to be most significant in the formation of a microcellular foams, where the large number of small well-distributed cells creates a situation in which the cell walls of the foamed material are significantly thinner that those of conventional foams.
There is a need in the art to provide foamed polymers which avoid the problems stated above, particularly with respect to semicrystalline polymers.
In one aspect, the invention provides a method of producing a foamed material. The method comprises contacting a mixture comprising a first thermoplastic polymer and a second thermoplastic polymer with a blowing agent. The first thermoplastic polymer has a higher percent crystallinity than the second thermoplastic polymer. The mixture of thermoplastic polymers is then subjected to conditions sufficient to create a thermodynamic instability in the blend to foam the mixture. The mixture comprising the first and second thermoplastic polymers has a percent crystallinity lower than the first thermoplastic polymer.
In another aspect, the invention provides a method of extrusion processing a blend of thermoplastic materials. The method comprises introducing at least two thermoplastic polymers into an extruder barrel. The at least two thermoplastic polymers comprise a first thermoplastic polymer and a second thermoplastic polymer. The first thermoplastic polymer has a higher percent crystallinity that the second thermoplastic polymer. The blend of thermoplastic materials is then heated to provide a molten blend thereof. The molten blend is then contacted with a blowing agent, and the blend is subjected to conditions sufficient to create a thermodynamic instability in the blend to foam the blend. The blend has a percent crystallinity lower than the first thermoplastic polymer.
Other aspects and advantages of the invention are set forth in detail herein.